Coating metal articles by powdered metals



Dec. 29, 1964 s. sToRcHHElM 3,163,527

COATING METAL ARTICLES BY POWDERED METALS Filed Oct. 4. 1960 Afm/WD@ United States Patent O 3,163,527 .A COATING METAL ARTICLES BY PCWDERED METALS Samuel Storchheirn, Forest Hills, N Y., assigner, by mestiza assignments, to Alloys Research 3: Manufacturing Corporation, a corporation of Delaware Filed Oct. 4, 1960, Ser. No. A60,374 5 Claims. (Cl. 75-206) The present invention is directed to the Yfabrication of metal powders, and more particularly to increasing the corrosion resistance of the products yobtained thereby.

It has been customary to protect metals against oxidation or other corrision by coating thereon a film of a more resistant metal or by producing a film of oxide on the Vbase metal. These methods have the disadvantage that the film may be damaged or ruptured in a minor degree exposing the base metal to corrosive elements. In the case of films which are brittle, the difficulty was aggravated.

The present invention is intended and 'adapted to overcome the difficulties and disadvantages inherent in the prior art, it being among the objects thereof to provide a process for coating films on metal powders which is simple, effective, and which will provide a high degree of v corrosion resistance to the finished product.

It is also among the objects of the invention to so treat metal powders that the products have substantially greater corrosion resistance than that normally obtained in castwrought products and other products of the same metals and made by conventional metallurgical techniques.

In practicing the invention the process involves coating metal powders with an oxide or other corrosion resistant film and then hot working the powders into the final shape desired. The properties of the coating on the powder particles is such that they remain partially intact after consolidation. As a result, the final powder product consists essentially of a honeycomb of oxide or other corrosion resistant material, the cells of which are filled with metal. Such a structure resists corrosive attack better than a cast-wrought material because the oxide, or other powder particle coating, is held rigidly in place by the metal matrix. Thus, it cannot spall or otherwise be removed as readily as cana coating on a flat surface. If the surface film around an individual cell should be removed accidentally, only the material contained within that cell will be corroded away, corrosion stopping or being lessened when the corroding medium encounters a new cell wall.

A uniform oxide coating can be applied to the particles through use of a fluidized bed technique. Satisfactory coatings have been obtained on coarse iron powders by iluidizing the iron powders with air for labout l0 or 15 minutes at 450 to 550 C. For fine powders which tend to burn in contact with air, steam fluidization appears to be the best oxidant. The choice of fluidizing medium or oxidant will largely depend on the properties of the material to be coated. Once coated, the powders may be consolidated into strip by such low cost processes as hot powder rolling or continuous sintering and hot pressing. Complex shapes may be produced by hot pressing, cold pressing and sintering or hot explosive compacting.

Molybdenum disilicide coatings are used for preventing the oxidation of molybdenum at high temperatures. A1- though the disilicide provides satisfactory oxidation resistance at temperatures up to 3000 F., it is very brittle and sensitive to thermal and mechanical shock and will not elongate appreciably under stress. Thus, the disilicide tends to crack and spall readily, thereby rendering the coating ineffective ras a means of protecting the molybdenum. By coating individual molybdenum powder par ticles with disilicide in a fluidized bed and hot consolidating the coated powders, Ea highly oxidation resistant prodrICC uct could be obtained. Such a product would have theY honeycomb structure described earlier; the honeycomb consisting of a network of molybdenum dis'ilieide with the individual cells of the honeycomb being filled with molybdenum.

The invention is more fully described in conjunction with the accompanying drawing constituting a part hereof and in which like reference characters indicate like parts, and in which- FIG. lis a diagrammatic view of an apparatus for .conducting the film forming portion of the invention, and

FIG. 2 is a chart showing the improved corrosion resistance -of the present products compared to prior products.

Referring Vto FIG. 1, there is provided a vertical tube 1 having a perforated plate 2 in the bottom thereof. A bed of iron powder 3 is placed on plate 2 to a suitable depth. A supply of air or steam through valved pipe 4 enters tube 1 through bottom 5. The rate at which the air or steam is introduced is sufficient so that the bed of powder 3 is quiescently fiuidized and excess entrainrnent of 'nesdoes not occur. The tube is in furnace 6 maintained at a temperature of about 450 to 550 -C. In about 5 'to l0 minutes the powder takes on a bright blue coating. Tube 1 then removed from the furnace and cooled by continuing the passage of air which flows out at '7 at the top of the tube.

To consolidate the oxidixed powders, they are loaded into a steel sheath fabricated from a 11A"O.D. tube 7 inches in length, flattened to in height, andrhot rolled in a 6" x l0 mill operating at 50 fpm. The rolling sequence is .as follows:

Percent Reduction Preheat Temperature, C.

Pass Number of the material fabricated from the oxide coated particles shows some 50% less corrosive attack than does the most corrosion resistant commercially available cast-wrought material; Armco iron.

Numeral S designates the product of oxide coated iron powder which has been subjected to 60% hot reduction and 9 where the reduction was 90%. At 10 is shown the results of an uncoated iron powder. The curve 11 is that of cast-Wrought Armco iron and 12 that of cast-wrought carbon steel. The improved corrosion resistance of the present product is readily apparent.

Materials having much better corrosion or oxidation resistance can be produced by this process than by other means. Thus, unalloyed material such as pure viron or steelV may be produced with corrosion resistant properties approaching that of stainless steel. Iron powders can be converted into sound strip having 50 percent better corrosion resistance than that of purest commercially avail-y able cast-wrought iron, such as Armco. The process is low in cost because such easily applied coatings as oxides the finished product.

ateaeav The corrosion resistance of materials fabricated in accordance with this invention is built in and cannot wear off, as in the case of plated materials. Nor is corrosion resistance eected by thermal and mechanical shock or stresses due to applied loads. This is of particular importance in applications where brittle coatings are required to provide the necessary resistance to oxidation or corrosion.

Sheared edges of strip fabricated by the present process have the same corrosion resistance as the faces. This is not true of plated materials since shearing the plate exposes the base metal to corrosion. The process is applicable to virtually any material on which an oxide can be formed or in which a corrosion resistant material can be deposited. Coating of the powders also improves the mechanical properties of the finished product, particularly the high temperature properties, since the dispersion of oxide in the base metal prevents movement of dislocations through the metal matrix, thus increasing the ultimate tensile strength and creep resistance of the final product.

Although the invention has been described setting forth a single specific embodiment thereof, the invention is not limited thereto as other metal powders may be used and other protective coating may be placed thereon. Various types, sizes and shapes of products may be made in accordance with the invention.

I claim:

1. AY method for fabricating iron-based material into parts having a relatively high degree of corrosion-resistance, comprising the steps of heating iron-based material in particulate form at a Yrelatively high temperature while exposing it to an oxidizing medium to encase the particles thereof with a protective coating formed by iron oxide, and subjecting the coated particles to pressure to partially rupture the iron-oxide coating on the particles and to consolidate the same into a body effectively constituted by a multi-cellular structure whose walls are formed by said coating and whose interconnecting cells are filled by said iron-based material,

2. A method for fabricating iron-based material into solid parts having a relatively high degree of corrosionresistance, comprising the steps of heating ironbased material in particulate form in a temperature range of about 450 C. to 550 C. while flowing an oxidizing medium through said material to encase the particles thereof with a protective coating formed by iron oxide, and subjecting the coated particles to heat and sintering pressure to partially rupture the iron-oxide coating on the particles and to consolidate the material into a solid body constituted by a multi-cellular structure whose walls are formed by said coating and whose interconnecting cells are filled by iron-based material.

3. The method set forth in claim 2, wherein said oxidizing medium is steam.

4. The method set forth in claim 2, wherein said sintering takes place in a temperature range of about 1000 C. to 1200 C.

5. A method for fabricating iron powder into solid parts having a relatively high degree of corrosion-resistance, comprising the steps of heating said powder while flowing stem therethrough to encase the individual powders with a bright blue iron-oxide coating, and subjecting the coated powders to heat and pressure to partially rupture the iron-oxide coating on the particles and to consolidate the material into a solid body effectively constituted by a multi-cellular structure whose walls are formed by said iron-oxide coating and whose interconnecting cells are filled with iron.

References Cited in the file of this patent UNITED STATES 4PATENTS 2,100,537 Conway Nov. 30, 1937 2,187,589 Lenel Ian. 16, 1940 2,809,891 Ennor Oct. 15, 1957 2,864,734 Adams et al Dec. 16, 1958 3,000,734 Grant 4et al. Sept. 19, 1961 

1. A METHOD FOR FABRICATING IRON-BASES MATERIAL INTO PARTS HAVING A RELATIVELY HIGH DEGREE OF CORROSION-RESISTANCE, COMPRISING THE STEPS OF HEATING IRON-BASED MATERIAL IN PARTICULATE FORM AT A RELATIVELY HIGH TEMPERATURE WHILE EXPOSING IT TO AN OXIDIZING MEDIUM TO ENCASE THE PARTICLES THEREOF WITHA PROTECTIVE COATING FORMED BY IRON OXIDE, AND SUBJECTING THE COATED PARTICLES TO PRESSURE TO PARTIALLY RUPTURE THE IRON-OXIDE COATING ON THE PARTICLES AND TO CONSOLIDATE THE SAME INTO A BODY EFFECTIVELY CONSTITUTED BY A MULTI-CELLULAR STRUCTURE WHOSE WALLS ARE FORMED BY SAID COATING AND WHOSE INTERCONNECTING CELLS ARE FILLED BY SAID IRON-BASED MATERIAL. 